The degree of monolithic integration has progressed very far in the field of radios. Under the name TDA 7340, SGS-THOMSON has brought out a monolithic integrated circuit which performs the entire audio signal processing of a stereo radio. For its operation, this integrated circuit resorts to a number of external circuit components, predominantly capacitors which cannot be monolithically integrated, or only with very great effort. For connecting these external circuit components this integrated circuit requires an accordingly high number of terminal pins.
In the input area of such an audio signal processing circuit, which includes a circuit device for selecting one of the signal sources and a signal amplifier with adjustable gain, there are circuit components with offset, namely the stereo decoder and an operational amplifier. This operational amplifier is followed by a circuit configuration for tone and volume control of the signal amplified by the operational amplifier. In the now customary way one uses, both for the gain adjustment of the operational amplifier and for the tone and volume adjustment by means of the following tone and volume control circuit, voltage dividers to be adjusted by means of digital control signals in the form of resistor strings with a plurality of component voltage taps. Unlike the formerly used potentiometers, such voltage dividers involve no continuous adjustability, but rather quantum adjustability. When such voltage dividers are supplied audio signals superimposed by a dc component due to previous circuit configurations with offset, this component is also stepped down in quanta by the digitally controllable voltage dividers. Upon a change of adjustment of such a voltage divider, this results in an audible click of the loudspeaker of a radio equipped with such an audio signal processing circuit.
In the known audio signal processing circuit, this problem has been overcome by connecting a capacitor in series between the output of the operational amplifier whose signal input is connected with the selected signal source, and the following tone and volume control circuit, thereby obtaining a dc decoupling and thus an elimination of the offset dc component.
By a multiplicity of measures the applicant has succeeded in providing a new monolithic integrated circuit for audio signal processing which manages with considerably fewer external circuit components than hitherto necessary, which has led to a saving of 24 terminal pins over the integrated circuit TDA 7340. This entails a considerable cost saving, on the one hand because a simpler housing can be used for the integrated circuit, and on the other hand because so many external circuit components can be dispensed with.
In the course of realizing this new integrated circuit, the desire arose to do without the external capacitor used for suppressing the offset voltage in the known signal processing circuit. For a radio suitable for stereo reception and therefore having two signal processing channels, this means a saving of two external capacitors and four terminal pins of the integrated signal processing circuit.
The present invention was therefore intended to eliminate the necessity of such external capacitors and the terminal pins required therefor and thus provide an offset compensating method which need not resort to external circuit components.